mineral

Black Opal

Amorphous Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs scale; Color: Dark body tone (N1-N4) with play-of-color; Luster: Sub-vitreous to waxy; Structure: Amorphous (non-crystalline); Cleavage: None; Specific Gravity: 1.9-2.3

Hardness: 5
Color: Dark body tone (N1-N4) with play-of-color
Luster: Sub-vitreous to waxy

Identified March 13, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 Mohs scale; Color: Dark body tone (N1-N4) with play-of-color; Luster: Sub-vitreous to waxy; Structure: Amorphous (non-crystalline); Cleavage: None; Specific Gravity: 1.9-2.3

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary rocks (typically ironstone or sandstone). Over millions of years, the water evaporates, leaving behind silica spheres. Most high-quality black opals formed during the Cretaceous period.

Uses & applications

Primarily used in high-end jewelry as gemstones, often cut into cabochons to display the play-of-color. They are also highly sought after by mineral collectors.

Geological facts

Black opal is the rarest and most valuable variety of opal. The 'play-of-color' is caused by the diffraction of light as it passes through regularly arranged microscopic silica spheres. Lighting Ridge, Australia, is the world's most famous source.

Field identification & locations

Identify by a dark body tone ranging from black to dark gray, combined with vibrant internal flashes of color. In the field, look for 'potch' (common opal) associated with color veins in sedimentary layers. Found mostly in Lightning Ridge (Australia) and parts of Ethiopia.